1. Field of the Invention
The present invention relates generally to methods and systems for determining the mass flow rate of a fluid, and more particularly to the operation of mass flow controllers and mass flow meters.
2. Discussion of the Related Art
Many industrial processes require precise control of various process fluids. For example, in the semiconductor industries, mass flow meters (MFMs) are used to precisely measure the amount of a process fluid that is introduced to a process chamber. In addition to measuring the mass flow, mass flow controllers (MFCs) are used to precisely measure and control the amount of a process fluid that is introduced to a process chamber. It is to be understood that the term fluid as used herein applies to any type of gas or vapor to which controlled flow may be of interest.
Mass flow controllers and mass flow meters have well defined performance criteria that are guaranteed by the MFC suppliers. Customer processes are based on repeatable performances from the devices after initial installation and process tune up. Mass flow controllers and mass flow meters that drift (e.g. due to aging) or get clogged (e.g. due to contamination) will cause a loss of repeatability that causes the process characteristics to change and will lead to lower yield or even total loss of the product being manufactured.
One typical implementation to detect this loss of performance is to make a rate of decay (ROD) measurement while the mass flow controller is flowing at a given setpoint to verify that the device characteristics have not changed. Starting a rate of decay measurement is done by interrupting the gas supply upstream of the pressure sensor and letting the pressure decay. Actual flow is directly proportional to the slope of the pressure decay. This process is performed off-line, meaning it requires the tool (i.e., the device utilizing the MFC or MFM) to stop processing wafers while the test is running. Since this impacts the customer's profits, it is not done very often, which could lead to running a process with low yield for an extended period of time.
To combat one or more of these problems, the disclosed embodiments include a system and method for providing a self-validating mass flow controller or mass flow meter. This will enable the mass flow controller or mass flow meter to provide real time information without requiring down time to make the measurements.